This invention relates generally to a method and apparatus for producing high quality continuous tone and/or color images on photosensitive material (i.e., photographic paper or film) from information provided in digital form.
In the field of photographic digital printers and image setters, the use of multiple light sources to create individual pixels is well known. U.S. Pat. No. 3,988,742 describes using LEDs and fiber optic light guides to deliver the light to the photosensitive material. Applications of this technology have included type setting, and the generation of lithographic films for printing. In these applications, the light output of LEDs is coupled into the input end of the fiber optic tubes. The output ends of the fiber optic tubes are arranged in a linear array. As photosensitive material is passed by the linear array of fiber optic tubes, the LED""s are illuminated in such sequence as to cause the formation of indicia or images on the photosensitive material. This process is described in U.S. Pat. Nos. 3,832,488 and 4,000,495 and 5,093,682. The use of fiber optic tubes of both square and round cross sections is known.
In such applications as described above, the precision assembly of the output ends of the fiber optics is important. Poor alignment, or uneven spacing of the output ends of the fibers cause distortions in the images being generated. U.S. Pat. Nos. 4,364,064 and 4,389,655 describe devices for precisely positioning the fiber optic tubes. U.S. Pat. No. 4,590,492 describes a method for masking the ends of the fiber optic tubes to provide more precise alignment of the light sources exposing the photosensitive material.
Prior art systems have typically been used to form lithographic images and indicia for typesetting and printing consisting solely of white and black areas without intermediate tones. Due to the lack of intermediate tonal detail, such systems are tolerant of some imprecision in the quality and quantity of light delivered to the photosensitive material. More particularly, they are typically tolerant of imperfect pixel to pixel alignment because of slight pixel blooming which occurs as a consequence of using exposure levels high enough to saturate the photosensitive material.
Continuous tone images, e.g., images which are predominately composed of middle tones, whether colors or gray tones, require significant precision in pixel formation and alignment. Misalignment of one pixel relative to its neighbors will cause unwanted lines or other artifacts to appear in a continuous tone image.
The present invention is directed to a method and apparatus for exposing photosensitive material to form high quality continuous tone, color images thereon.
Embodiments of the invention are typically comprised of an imaging (or print) head comprised of multiple pixel image generators, e.g., light sources. The head is preferably mounted for linear movement in a first, i.e., scanning direction, across the width of a web of photosensitive material. The photosensitive material is mounted for linear movement in a second direction perpendicular to said first direction to enable successive scan strips (i.e., groups of scan lines) to be imaged onto said photosensitive material. The portion of the web to be printed can be referred to as an xe2x80x9cimage fieldxe2x80x9d and can be considered to consist of a rectangular matrix of xe2x80x9crowsxe2x80x9d extending in the scan direction across the web width and xe2x80x9ccolumnsxe2x80x9d extending perpendicular to the rows, i.e., longitudinally along the web. The head can be stepped or moved continuously in the scan direction with the multiple light sources being selectively enabled to expose an image onto the photosensitive material.
A system in accordance with the invention produces a field of xe2x80x9cinterpolated pixelsxe2x80x9d, each interpolated pixel being formed by the overlap between adjacent pixel images. More particularly, an interpolated pixel in accordance with the invention can be formed by the overlap between adjacent pixel images displaced in the scan direction, e.g., horizontal, and/or by the overlap between adjacent pixel images displaced in the longitudinal direction, e.g., vertical.
It is an object of this invention to provide a method and apparatus for precisely delivering light to photosensitive material to allow the printing of extremely high quality continuous tone images from digital information.
It is a further object of this invention to provide a method and apparatus for blending the pixels of an image presented in digital form, so as to increase the apparent resolution and sharpness of the resulting printed image.
It is a further object of this invention to provide a low cost imaging head capable of precisely delivering light to photosensitive material with the precision required to allow the printing of extremely high quality continuous tone images.
In accordance with a preferred embodiment of the invention, the imaging head is comprised of square or rectangular pixel image generators, e.g., fiber optic tubes, mounted to form a rectangular array. The pixel image generators are inclined at an angle of 45 degrees to the scan direction. As the print head scans, each fiber optic tube can expose a pixel image onto the photosensitive material. The exposure levels of the pixel images are preferably specified in a digital file representing an image to be printed. As a result of being inclined at 45 degrees, the pixel images exposed onto the photosensitive material are diamond shaped. Each pixel image overlaps its neighbor by substantially 50% of the center to center distance between pixel images. The pattern resulting from the overlapping of the pixels images generates geometrically interpolated pixel areas with each such area being about 25% of the original pixel image area. Further, the shape of each pixel relative to the scan direction causes the exposure in the area between adjacent scan lines to remain consistent and to cause superior blending of each pixel image with its neighboring pixel images.
The generation of high quality continuous tone images requires the precise blending of the pixels imaged on the photosensitive material. Precise blending of the pixels requires that the pixels themselves be of uniform color and intensity. Fiber optic tubes operate by the principle of xe2x80x9ctotal internal reflectionxe2x80x9d of the light waves introduced into the fiber optic tube. The symmetric nature of round fiber optic tubes is such that the image of the light source at the input end of the tube is delivered, more or less intact, to the output end of the tube. The nature of LEDs and other light sources is that the light emitting element does not emit perfectly uniform illumination. Consequently, the image of the LED die will propagate down a round fiber optic tube and be delivered to the end. The non-uniform nature of the image of the light element will cause the pixels to blend poorly with one another. In a fiber optic tube of square or rectangular cross section, the image is scrambled by the successive reflections off the orthogonal walls of the tube. The principles of xe2x80x9ctotal internal reflectionxe2x80x9d causes most all of the light energy which enters the fiber optic tube to be delivered to the output, but with the image scrambled to such an extent as to make the light output from the fiber optic tube substantially uniform. This uniformity is desirable to achieve the highest quality of continuous tone images.
As described above, the generation of high quality continuous tone images requires extreme precision in the placement and uniformity of illumination of the pixels comprising the image. Imprecision in either of these will result in artifacts or lines appearing in the printed image. In the printing method as described herein, a plurality of independently excitable light sources is employed to provide the illumination for a matching number of pixels. When printing onto color photosensitive materials, extreme precision is required in the matching of the spectral output characteristics of the multiple light sources. If the light sources are not of precisely the same spectral characteristics, artifacts or lines will appear in the printed image. Light sources of different spectral characteristics will expose different layers of the photosensitive material with differing efficacy. It is possible to adjust the intensity of light sources to be equally effective in exposure at a particular color or shade. However, if the spectral characteristics of the light sources are not precisely matched to one another, they will not be equally effective at exposing a different color. The result will be that artifacts or lines appear in some colors of the printed image, but not others.
In a preferred embodiment of the invention, the individual light sources are matched in spectral output with the use of a narrow pass band filter for each color. The narrow pass nature of the filter restrains the exposing energy of each LED to a narrow wavelength range within which the photosensitive material will have uniform color response. The filter is fabricated in such a way as to cover all of the pixels of a given color with the same filter. In the embodiment of the invention actually constructed by the inventor, filters of two independent wavelengths were fabricated on the same substrate and placed over the ends of the fibers.
The print head can be imaged onto the photosensitive material either by intimate contact or via a lens system. In one implemented embodiment of the invention, the print head is comprised of three columns of fiber optic tube ends, each column containing 32 tube ends. The head scans across a 30 inch width of photosensitive material and exposes a strip of approximately 100 inches during each scan. After each scan, the material is advanced in the longitudinal direction by the height of the exposed strip area. The head then successively scans across the photosensitive material exposing additional strip areas to fully cover the image field. In an alternative embodiment of the invention, the head could be the full width of the material obviating the need for the head to scan across the width of the material.